Wireless Localisation System

1. An apparatus for estimating the location of a remote node, the apparatus comprising: an antenna array comprising a plurality of elements in a fixed spatial arrangement, at least one element being a transmitting element configured to transmit a first wireless signal to the remote node, and at least two elements being receiving elements configured to receive a second wireless signal transmitted by the remote node in response to the first wireless signal, wherein the first wireless signal is used to measure a first time-of-arrival between the apparatus and the remote node and the second wireless signal is used to measure a second time-of-arrival between the remote node and the apparatus; and a signal processing unit connected to the antenna array, the signal processing unit being configured to: estimate a plurality of round trip distances using the first time-of-arrival and second time-of-arrival based on the first and second wireless signals, each round trip distance being from the transmitting element to the remote node and back to the receiving element; and estimate the location of the remote node using the round trip distance estimates; wherein each receiving element is configured to measure the phase of the second wireless signal relative to a common reference phase, and the signal processing unit is configured to estimate the location of the remote node using the phase measurements as well as the round trip distance estimates.

2. An apparatus according to claim 1 , wherein the location estimating comprises solving a linear equation relating the location of the remote node to the round trip distance estimates via an array matrix.

3. An apparatus according to claim 2 , wherein the solution is a least squares solution.

4. An apparatus according to claim 1 , wherein: each receiving element is configured to measure the time-of-arrival of the second wireless signal relative to a common clock, the remote node is configured to measure the time-of-arrival of the first wireless signal from the transmitting element, and the signal processing unit is configured to estimate the round trip distances using the measured times-of-arrival.

5. An apparatus according to claim 1 , wherein each receiving element is a transmitting element, and each round trip distance is from an element to the remote node and back to that element.

6. An apparatus according to claim 1 , wherein each round trip distance is from a transmitting element to the remote node and back to a receiving element.

7. An apparatus according to claim 1 , wherein the location estimating comprises jointly solving a linear equation relating the location of the remote node to the round trip distance estimates via an array matrix, and a linear equation relating the location of the remote node to the phase measurements via a phase array matrix.

8. An apparatus according to claim 7 , wherein the solution is a weighted least squares solution, weighted by the variance of the errors in the round trip distance estimates, and the variance of the errors in the phase measurements.

9. An apparatus according to claim 1 , wherein the signal processing unit is further configured to resolve the ambiguity in the phase measurements.

10. An apparatus according to claim 9 , wherein the ambiguity resolving comprises: estimating a bearing of the remote node using the round trip distance estimates; computing, for each receiving element, an integer that embodies the phase ambiguity using the bearing estimate, wherein the estimating the location of the remote node uses the computed integers to resolve the ambiguity in the respective phase measurements.

11. An apparatus according to claim 9 , wherein the ambiguity resolving comprises: estimating a bearing of the remote node using the round trip distance estimates; finding a plurality of peaks in the array pattern around the bearing estimate; computing, for each peak, and for each receiving element, an integer that embodies the phase ambiguity using the bearing estimate; forming, for each peak, an estimate of the location of the remote node using the computed integers to resolve the ambiguity in the respective phase measurements, and a corresponding error; choosing the location estimate yielding the smallest error.

12. An apparatus according to claim 1 , wherein the location estimating comprises minimising a cost function of the range and bearing of the remote node, the parameters of the cost function being the round trip distance estimates and the location of each receiving element.

13. An apparatus according to claim 1 , wherein the location estimating comprises minimising a cost function of the range and bearing of the remote node, the parameters of the cost function being the round trip distance estimates, the phase measurements, and the location of each receiving element.

14. A method of estimating a location of a remote node, the method comprising: estimating a plurality of round trip distances, each round trip distance being from a transmitting element to the remote node and back to one of a plurality of receiving elements based on a first wireless signal transmitted by the transmitting element to the mobile node and a second wireless signal transmitted by the mobile node to the receiving element, wherein the first wireless signal is used to measure a first time-of-arrival between the apparatus and the remote node and the second wireless signal is used to measure a second time-of-arrival between the remote node and the apparatus; estimating the location of the remote node using the first time-of-arrival and second time-of-arrival based on the first and second wireless signals to calculate the round trip distance estimates; measuring the phase of the second wireless signal relative to a common reference phase; and estimating the location of the remote node using the phase measurements as well as the round trip distance estimates.